The present invention relates to a cermet alloy which is superior in resistance to high temperature wear, high temperature strength and chipping resistance.
In general, a known cermet alloy contains hard titanium carbide (expressed as "TiC" hereinafter) as the main constituent, and a component such as molybdenum carbide, tungsten carbide, tantalum carbide or niobium carbide (respectively expressed as "Mo.sub.2 C, WC, TaC and NbC" hereinafter) which is added in order to improve wettability between a bonding phase which is composed of a metal and TiC grains or grains of titanium carbo-nitride (expressed as "TiCN" hereinafter) which are hard grains similar to the TiC grains. Such an additive component is dissolved into the bonding phase and precipitates around the TiC or TiCN grains during sintering at high temperature, so as to envelope the TiC and TiCN grains thereby forming a surrounding structure, thus contributing to improvement in the wettability to the bonding phase. In the known cermet alloy, therefore, the composite carbo-nitride of the type mentioned above has a double core structure, wherein the core structure is rich in titanium (expressed as "Ti" hereinafter) while the surrounding structure is rich in Mo.sub.2 C, WC, TaC or NbC which improves the wettability between the hard grains and the bonding phase which is not rich in Ti. Such a cermet alloy is disclosed, for example, in Japanese Patent Publication No. 56-51201 and Japanese Patent Laid-Open Publication Nos. 61-73857, 61-201750 and 61-210150.
FIG. 7 is a scanning electron microscopic photograph of the micro structure of this known cermet alloy. It will be seen from this Figure that the core structure of the double core structure of the composite carbo-nitride is dark thus suggesting that this core structure is rich in Ti which is a light element, while the surrounding structure is bright thus suggesting that this portion is rich in heavy elements such as tungsten (expressed as "W"), tantalum (expressed as "Ta") and so forth.
On the other hand, an analysis of the composite carbo-nitride in the double core structure through a transmission analyzing electron microscope showed that the core structure contains 65.8% of Ti and 5.0% of W while the surrounding structure contains 49.5 wt % of Ti and 23.2 wt % of W. Thus, the core structure is rich in Ti and poor in W, while the surrounding structure is rich in W and poor in Ti as compared with the core structure.
When the known cermet alloy having the above-described micro structure is used as a material of a cutting tool for high-speed cutting, the binding phase having a comparatively small hardness is worn so that the composite carbo-nitride grains appear on the surface of the tool. However, the surrounding structure of the double core structure, which is rich in W and poor in Ti, exhibits a large oxidation tendency and low hardness, to thereby be worn rapidly. In consequence, it is impossible to fully utilize the advantage offered by the Ti as the hard component. In addition, the surrounding structure is constituted by components such as Mo.sub.2 C, WC, TaC and NbC which are intended for improving wettability to the bonding phase, so that the composite carbo-nitride grains grow during the sintering with the result that the grown grains are in contact with each other. Obviously, the bonding strength is small in the regions where the composite carbo-nitride grains contact each other so that fine cracks tend to appear at these regions when an external stress is applied In addition, these regions tend to cleave as paths of propagation of the cracks. In consequence, the fracture toughness value of the cermet alloy is reduced and chipping resistance is also impaired as the number of regions of mutual contact of grains becomes greater. These problems would be overcome by reducing the contents of the components constituting the surrounding structure. In such a case, however, the high temperature strength of the cermet alloy is seriously degraded. Therefore, it is necessary to maintain a considerably large content of the components constituting the surrounding structure. In consequence, the cermet alloy inevitably has a considerably large number of regions where the composite carbo-nitride grains contact one another.
In order to overcome the above-described problems, it has been proposed to disperse a pseudo-TiC phase which is rich in TiC besides the composite carbo-nitride phase, so as to improve the wear resistance, as disclosed in Japanese Patent Laid-Open Publication No. 61-199048.
Also it has been proposed to form an alloy in which the hard phase has a two-phase structure composed of both an NaCl-type solid solution phase with a core and titanium nitride (expressed as "TiN") and in which fine grains having a composition of Ni.sub.3 Al(Ti) composition is precipitated and dispersed in the bonding phase, as disclosed in Japanese Patent Laid-Open Publication No. 63-39649.
The known cermet alloy of the type described is still unsatisfactory in that the wear resistance is not so high, although the proposal in Japanese Patent Laid-Open Publication No. 61-199048 offers an appreciable improvement in the wear resistance. In this improved cermet alloy, however, the composite carbo-nitride grains other than the pseudo-TiC phase still have surrounding structure similar to that in the conventional cermet alloys In addition, the composite carbo-nitride grains other than the pseudo-TiC phase occupies most portion of the hard phase. Therefore, when the hard phase has appeared on the tool surface after wear of the bonding phase which has a comparatively small hardness, no substantial improvement is achieved in the wear resistance insofar as the the composite carbo-nitride grains other than the pseudo-TiC phase have surfaces rich in W and poor in Ti as in the case of the conventional cermet alloys. Furthermore, the addition of the pseudo-TiC phase cannot significantly increase the wear resistance considering that this phase occupies only 20 vol % or so of the whole hard phase, though this phase exhibits a comparatively high hardness.
The proposal made in Japanese Patent Laid-Open Publication No. 63-39649 encounters a problem substantially the same as that explained in connection with Japanese Patent Laid-Open Publication No. 61-199048. It is true that the cermet alloy disclosed in Japanese Patent Laid-Open Publication No. 63-39649 has a comparatively large TiN content. The TiN, however, is partly dispersed in a NaCl type solid solution and partly exists as independent TiN phase. The independent TiN phase occupies only a small part of the hard phase and, therefore, is expected to produce only a small effect on the improvement in wear resistance. It is stated that the strength of the bonding phase is improved by allowing dispersed precipitation of fine grains having a composition of Ni.sub.3 Al(Ti). The dispersion of the fine grains in the bonding phase is effected by allowing precipitation in the course of the sintering, while the bonding phase is composed of nickel and aluminum (expressed as "Ni" and "Al", respectively) or Ni and cobalt (expressed as "Co"). It is therefore extremely difficult to control mean grain size, precipition amount and other factors, as well as the trace amount of Al to be added.